Aqueous dispersions of phenolic resins are becoming more important commercially in many end-use areas, such as coatings, adhesives, fiber bonding, and the like. An important advantage of such aqueous dispersions is the reduction in environmental pollution that is obtained because the use of organic solvents is reduced or eliminated, and/or the proportion of unreacted phenol and/or formaldehyde in the phenolic resins used in aqueous dispersions is sharply reduced.
Two different approaches have been used to produce aqueous phenolic resin dispersions. One is basically an in situ preparation wherein the phenol and aldehyde (usually formaldehyde) are reacted in an aqueous medium, and are dispersed therein with the aid of an interfacial agent without having been isolated from the aqueous reaction medium. Examples of such in situ preparations include Harding, U.S. Pat. No. 3,823,103 and Ingram, U.S. Pat. No. 3,666,694.
The second approach utilizes a pre-formed phenolic resin which is dispersed in water in various ways, typically, either by first dissolving the resin in an organic solvent and emulsifying the solution with the help of a surface active agent, or by pulverizing the resin to a very fine particle size and dispersing it in water with a surface active agent. Both approaches suffer from the disadvantage of requiring the use of a surfactant. This can be disadvantageous in many end-use applications, e.g., coatings, because water resistance may be impaired. Dispersions made by using pulverized phenolic resin tend to be gritty, which makes them unsuitable for surface coating applications.